In an engine control of a diesel engine (hereinafter referred to as “engine”) used for a work machine, when an operator of the work machine optionally sets a fuel adjustment dial (throttle dial) provided in an operator room, an engine controller outputs a control signal to inject fuel into the engine, at an amount according to the setting, to a fuel injection system. So as to keep an target engine speed set by the fuel adjustment dial (throttle dial), the engine controller outputs a control signal corresponding to the change in load of the work apparatus attached to the work machine to the fuel injection system so as to adjust the engine speed. The engine controller or a pump controller calculates a target absorption torque of the hydraulic pump according to the target engine speed. The target absorption torque is set so as that the engine output horsepower is in balance with the absorption horsepower of the hydraulic pump.
Typical engine control will be described using FIG. 24. The engine is controlled so as not to exceed an engine output torque line TL determined by a maximum output torque line P1 of the engine and an engine droop line Fe drawn from the maximum engine speed. For example, when the work machine is an excavator or the like, the engine controller generates a control signal for changing the engine speed according to the manipulation amount of a manipulating lever which is manipulated for swinging an upper swing body or operating the work apparatus and the load of the work apparatus or the like. For example, when an operation of excavating earth, sand, or the like is carried out with the target engine speed set at N2, changeover is made from the engine speed at which the engine idles (idling engine speed N1) to the target engine speed N2. In this changeover, the fuel injection system, by receiving the control signal from the engine controller, injects fuel into the engine according to the changeover, and thereby the load increases by the operation of the work apparatus or the like. Thereby, the engine speed changes over so as that the engine speed and the engine output torque reach a matching point M1 corresponding to the intersection point of the pump absorption torque line PL of a variable displacement hydraulic pump (typically, a swash plate type hydraulic pump) and the engine output torque line TL. Note that, the engine output is maximum at the rated point P.
As illustrated in FIG. 25, in order to improve fuel consumption efficiency of the engine and pump efficiency of the hydraulic pump, an engine control apparatus in which a target engine operating line (target matching route) ML which runs through the region where fuel consumption rate is preferable is provided, and a matching point of the engine output and the pump absorption torque is provided on the target matching route ML. In FIG. 25, the curve M illustrates a constant fuel consumption curve of an engine, and the fuel consumption rate is better in the region close to the center of the curve M (eye M1). Further, a curve J illustrates a constant horsepower curve in which the horsepower absorbed by the hydraulic pump is constant horsepower. Therefore, regarding cases in which the same horsepower is obtained, the fuel consumption rate is better in the case when the matching is made at a matching point pt2 on the target matching route ML than in the case when the matching is made at a matching point pt1 on the engine droop line Fe. Further, the flow rate Q of the hydraulic pump is the product of the engine speed n and the pump capacity q (Q=n·q). Therefore, to obtain the same work fluid flow rate, pump efficiency is better when the engine speed is reduced so as to increase the pump capacity.